Keywords: Li 4 Ti 5 O 12 ; Sol-gel; Hydrothermal; Li-ion battery; Anode material; Sintering; TiO 2 xerogel.

Transcription

1 Synthesizing Lithium Titanate (Li 4 Ti 5 O 12 ) for Li-ion Battery Anode material by Hydrothermal Process using Lithium Hydroxide (LiOH) and Titanium Dioxide (TiO 2 ) Xerogel Keywords: Li 4 Ti 5 O 12 ; Sol-gel; Hydrothermal; Li-ion battery; Anode material; Sintering; TiO 2 xerogel. ABSTRACT Lithium titanate, Li 4 Ti 5 O 12 /(LTO) is a very potential anode material for high performance lithium ion battery. In this work, LTO was synthesized by hydrothermal method using TiO 2 xerogel prepared by sol-gel method and lithium hydroxide. The sol-gel process was used to synthesize TiO 2 xerogel from titanium tetrabutoxide precursor. Anatase polymorph was obtained by calcining the TiO 2 xerogel at low temperature, i.e.: 300 o C and then reacted with 5M LiOH aqueous by hydrothermal process at 135 o C for 15 hours to form Li 4 Ti 5 O 12. Sintering process was used in temperature varied at 550 o C, 650 o C, and 750 o C to determine the optimum characteristic of Li 4 Ti 5 O 12 based on Scanning Thermal Analysis (STA), XRD, FESEM, FT-IR, and BET characterization. The highest intensity of XRD peaks and FTIR spectra of the LTO were found at the highest sintering temperature (750 o C). As a trade-off, however, the obtained LTO/Li 4 Ti 5 O 12 possesses the smallest BET surface area (~ 0 m 2 /g) and highest crystallite size (56.45 nm). Keywords: Li 4 Ti 5 O 12, Sol-gel, Hydrothermal, Li-ion battery, Anode material, Sintering, TiO 2 xerogel. 1. INTRODUCTION Various ideas have been introduced to overcome the energy crisis and environmental pollution. The most promising one is development of electric vehicle which has zero emission with the electric power to charge the car s battery comes from renewable energy source. Electric vehicles create additional economic development opportunities by improving quality of life, reducing energy spending, and decreasing reliance on foreign oil (Todd 2013). The fuel tank of the electric car is a rechargeable battery, in which the lithium ion battery (LIB) is the most widely used today. Graphite is commonly used as anode material for LIB, however, during intercalation process in graphite anode, the formation of dendritic structure may causes short-circuit and volume expansion (Wang et al. 2013), and the formation of solid electrolyte interphase (SEI) lead to decreasing in specific capacity (Jiang et al. 2014). Lithium titanate (LTO) offers some advantages, since LTO is known as a zero-strain insertion oxide (Mosa et al. 2012), it can experience thousands of cycles with little capacity loss and is able to undergo a fast charging (Wang et al. 2013). Additionally, solid electrolyte interphase (SEI) formation could be avoided since LTO s insertion potential is at 1.55 V (1.55 V vs. Li/Li+), which is well above the SEI formation potential (0.9V), offering the nanoscale features which is not normally feasible

2 with carbon electrodes due to excessive SEI build-up (Maloney et al. 2012). Further, LTO has the potential for electrochemical reaction kinetics leading to excellent rate capability as well as stable cycling performance due to its the electrochemical characteristics (Li & Mao 2014). In addition to its advantageous properties, LTO has major drawback due to its low electrical conductivity (Mosa et al. 2012), and therefore needs modifications for use in high current application. Thus, although the LTO possesses a theoretical specific capacity of 175 mah/g, the capability to provide charge/discharge currents are relatively low due to its large polarization resulting from the low electrical conductivity (Mosa et al. 2012) and slow Li-ion diffusion (Ouyang et al. 2007). Some efforts to enhance LTO conductivity have already been carried out by doping with other elements (Zhang et al. 2014) and coating using more conductive materials (Park et al. 2008). To increase the ionic diffusion of ion Li +, the LTO particle size has been reduced to increase the surface contact between electrolyte and electrode, and also shorten diffusion path of lithium ion and electron as means to improve the lithiation kinetics (C. Zhang et al. 2013). Sol-gel method has been applied extensively to synthesize nanoparticle due to its ability to control the reaction step at the molecular level to get the nanoparticle with high homogeneity (Bilecka & Niederberger 2010). The product resulted from sol-gel process has the characteristic of higher surface area compared to solid-state process (Priyono et al. 2013). Moreover, the sintering temperature required to form the structure of spinelcrystalline significantly is lower than that of solid-state method (C. Zhang et al. 2013). Further, hydrothermal technique is a method used for reacting two or more compounds in the autoclave with the aid of water vapor pressure over 1 atm and temperatures above 100 C (Byrappa & Yoshimura 2001). To improve the solid nanostructure, hydrothermal process aged at C for 1-4 days was reported to able to increase the degree of crystallinity and thermal stability by minimizing the grain growth grain growth up to 800 C (Wang & Ying 1999). In this work, LTO is synthesized using sol-gel procedure to prepare TiO 2, followed by hydrothermal method combined with solution impregnation mixing and sintering to form the spinel structure of LTO. Sol-gel process is used to prepare TiO 2 xerogel, followed with calcination to form the TiO 2 -anatase phase which is further mixed with lithium hydroxide solution in water and finally hydrothermally processed. Finally, to obtain the spinel LTO, the sintering process was varied at temperatures of at 550, 650 and 750 ºC. The whole process is aimed at obtained LTO with improved nano-crystalline structures to support its function as anode material in lithium-ion battery. 2. METHODOLOGY/EXPERIMENTAL Titanium tetra-n-butoxide/ti(oc 4 H 9 ) 4 (Kanto Chemical), Ethanol/C 2 H 5 OH (pa), HCl 1M, and LiOH (pa) were used in this work. Firstly, titanium tetra-n-butoxide was used to prepare TiO 2 xerogel which was further calcined at 300 ºC to obtain TiO 2 anatase. The procedure to obtain TiO 2 xerogel was explained in our previous work (Priyono et al. 2013). After xerogel was ground in mortar, the calcination was carried out at 300 C for 3 hours under airflow to remove the remaining organic compounds and to complete the formation of anatase TiO 2. The obtained TiO 2 powder was impregnated with aqueous LiOH (5M),

3 and then further hydrothermally treated in a Teflon-lined autoclave at a temperature of 135ºC for 15 hours. Finally, the sintering process was carried out at temperatures varied at 550, 650 and 750 ºC to obtain (Li 4 Ti 5 O 12 )/LTO. The off-white LTO powder was collected for further analysis. The characterization by x-ray diffraction/xrd (Bruker AXS θ 2θ diffractometer, Philips PW3020) was carried out to identify the crystal structure of the obtained solid samples. The crystallite size was estimated based on XRD data via Scherrer s equation analysis. Further, Brunauer-Emmet-Teller (BET) measurement (Quantachrome NOVA 1200e) was used to determine the surface area of the sample. FESEM analysis was performed to identify the surface morphology and particle size of LTO. Finally, FTIR (Shimadzu IR Prestige-21) analysis was carried out to verify the stretching bonds existing in the oxide network as means to identify the functional groups contained in the compounds (Sui et al. 2006). For comparison purposes, the LTO obtained by the courtesy of the KIST (Korean Institute of Science and Technology) was used in this research as reference. 3. RESULTS The xerogel powder prepared from sol-gel then calcined at 300 C for 3 hours under airflow obtains anatase TiO 2. BET surface area measurement showed that the obtained TiO 2 phase via sol-gel method and calcination at 300ºC has surface area of m 2 /g. Then sampel was impregnated with aqueous LiOH (5M), and hydrothermally treated in a Teflon-lined autoclave at a temperature of 135ºC for 15 hours. The sample obtained after hydrothermal process appeared as light brown solid. The XRD pattern of the post-hydrothermal solid is depicted in Figure 1 and the FESEM image of post-hydrothermal solid is shown in Figure 2. Figure 2. X Ray-Diffraction Pattern of anatase TiO 2 xerogel mixed with LiOH and hydrothermalled at 135ºC, 15 h. Figure 1. FESEM Micrograph of posthydrothermal solid at magnification 1,000X (2A), 2,000X (2B), 5,000X (2C) and 10,000X (2D). The solid products from hydrothermal processing are then subjected to sintering at temperature varied at 550, 650 and 750ºC. In Figure 3, it is shown the XRD pattern of the post-sintering solids together with LTO reference from KIST. The average crystallite size of the solids calculated from is presented in Table 1.

4 Table 1. Average crystallite size diameter Sintering Temperature Crystallite size C nm C nm C nm Figure 3. X Ray-Diffraction Pattern of the solid products sintered at temperature 550, 650 and 750ºC. The results of FTIR testing are shown as follows. Figure 4 shows the FTIR spectra of the xerogel TiO 2 and Figure 5 shows the spectra of the all produced solid at sintering temperature 550, 650 and 750ºC. Figure 4. FTIR spectra of the xerogel TiO2. Figure 5. FTIR spectroscopy of the solid product sintered at temperature 550 ºC, 650 ºC and 750 ºC. Furthermore, the result of BET surface area testing of sintered product is presented in Table 2. While in Figure 6, it is shown the results of SEM samples of all produced sintering of the solid at the sintering temperature of 550, 650 and 750ºC with X magnification. Table 2. BET surface area of sintered product. Temperature ( 0 C) Surface area (m 2 /g) ~ ~ 0

5 Figure 6. FESEM micrographs of post-sintering solid at magnification 10,000X, sintered at 550ºC (A), 650ºC (B), 750ºC (C), and LTO from KIST (D). 4. DISCUSSION The BET surface area measurement showed that the obtained TiO 2 phase via sol-gel method and calcination at 300ºC has surface area of m 2 /g, which is higher than other xerogel calcined at 420ºC (Priyono et al. 2013). Additionally, the surface area of the synthesized TiO2 xerogel in this work is significantly higher than those of commercial ones, i.e. from Merck (10 m 2 /g) (Augugliaro et al. 2005) and Sigma (18,75 m 2 /g) (Ruslimie et al. 2011), This confirms that the sol-gel method could synthesize a solid material with high surface area (Wen 2012). Thus, the present TiO 2 xerogel has the prospective for better starting material for manufacturing the high surface area LTO. Analogically, to obtain high surface area LTO, it must use a high surface area starting material, since upon the subsequent heat treatment process to obtain crystalline structure, the agglomeration in the solid cannot be avoided, and thus reducing the surface area. In view of the foregoing, it is important to use the lower temperature process during calcination and sintering in order to preserve the surface area of the solid (Priyono et al. 2015). From the result of hydrothermal treatment, as it can be seen from Figure 1, the diffraction pattern shows 4 compounds, i.e.: TiO 2 anatase, TiO 2 brookite, Li 2 TiO 3 and Li 4 Ti 5 O 12. The peaks of Li 2 TiO 3 and Li 4 Ti 5 O 12 are detected by software embedded in XRD instrument (X pert HighScore) with low match value, indicating that starting stage the crystallite seed of the both compounds to grow. Some of peaks of the compounds Li 2 TiO 3 and Li 4 Ti 5 O 12 are almost overlap due to 2θ value are very close, particularly for the plane of (440) of Li 4 Ti 5 O 12 and plane (312) of Li 2 TiO 3. Based on this result, it is shown that using xerogel TiO2 and LiOH precursor, the seed of Li 2 TiO 3 and Li 4 Ti 5 O 12 could be initiated after hydrothermal process. This could improve the result obtained by S. Suzuki et.al. (Z. Zhang et al. 2013), that that the hydrothermal process cannot synthesize the lithium titanate directly, but the precursor can be transformed to spinel Li 4 Ti 5 O 12 after heat treatment. The peak intensity of the compounds TiO 2 anatase, Li 2 TiO 3 and Li 4 Ti 5 O 12 looks similar. The crystallite size calculated via Scherrer s equation by Williamson-Hall Plot using fit-size analysis is obtained equal to 15.7 nm.

6 The FESEM micrograph from Figure 2 shows the particles of the solid in forms of irregular agglomerates with mean size measured using ImageJ software is 1.79 μm. However, this value may not fully represent the real situation due to several larger-sized particles were cut during image enlargement process in the instrument. The agglomeration of particles could be increased by existence of moisture during hydrothermal process (Hintz & Thomas 2014). The XRD result as shown on Figure 3, it can be seen that the XRD patterns of the solids which differ gradually from temperature 550, 650 and 750ºC. First, at the temperature 550 ºC, it founds the TiO 2 rutile (JCPDS: ) peak intensity is the strongest, followed by LTO (Li 4 Ti 5 O 12 ) spinel crystalline structure (JCPDS: ) and still detected small TiO 2 anatase (JCPDS: ) peak at 25.3 o. Then, at temperature 650 ºC, the smaller TiO 2 anatase peak is still observed and TiO 2 rutile peak intensity is also getting lower than that of at 550 ºC, while the LTO peak looks stronger. Finally, the outcome of sintering temperature at 750 ºC is that the LTO becomes a major phase, and rutile peaks getting lower significantly. However, the small peak of Li 2 TiO 3 (JCPDS: ) is detected at 43.6º. The above result shown that increasing temperature in the range of 550 to 750ºC is in favor of the formation of LTO. The rutile TiO 2 phase is present in all sintered products since the sintering temperature is under 800ºC (J.W. Shin et al. 2012). If the sintering temperature is above 800 ºC, it will result in grain growth and increasing the particle size diameter, and thus the agglomeration will be increased. The impurity of TiO 2 rutile is formed by unreacted TiO 2 anatase during reaction formation of LTO. It is noted at the sintering temperature 750ºC result, the existence of peaks Li 2 TiO 3 indicates that the unreacted TiO 2 anatase is caused not only by lost of Li + ion source during sintering process, but also by inhomogeneous mixture during solid state mixing process. The presence of Li 2 TiO 3 compound which has molar ratio of Li/Ti greater than Li 4 Ti 5 O 12, shows that not all Li + ions react uniformly with TiO 2 anatase thus it forms Li 2 TiO 3 and residual unreacted anatase TiO 2 will transform into rutile TiO 2 (Hong et al. 2012). Further, the low intensity with almost similar height of Li 2 TiO 3 and TiO 2 rutile peaks at the stoichiometric composition indicates that there is a little lost of Li + ion source during sintering process. Moreover, Li 2 TiO 3 compound is usually become one of major impurities in the synthesizing process of Li 4 Ti 5 O 12 compound. To get to single phase Li 4 Ti 5 O 12 free from impurities such as Li 2 TiO 3 is very difficult. As reported by Wen (Wen 2012), Li2TiO3 is always coexisted as impurity along with Li4Ti5O12 obtaining by solid-state method. It is noticeable that both Li2TiO3 and Li4Ti5O12 phases are layer structured, far from perfect, with their interplanar distances, Li2TiO3 (002) and Li4Ti5O12 (111), being very close to each other, i.e. 4.80Å and 4.84 Å, respectively. These two substances most probably interlock with each other, at the stage of their coexistence. Thus, by using better mixing process of rutile-tio 2 with Li + ions source in a stoichiometric composition, the existence of impurity Li 2 TiO 3 could be minimized. Further, as shown in Table 1, by using Scherer equation with Williamson-Hall fit size method calculation, it is obtained the crystallite size of the solid sintered at 550ºC is nm, at 650ºC is nm, and at 750ºC is nm, while the LTO KIST is nm. The average crystallite size is bigger in accordance with the increasing sintering temperature. Thus, it is such an indication that this hydrothermal method using TiO 2 anatase prepared by

7 sol-gel to produce LTO and the sintering temperature of 750ºC is sufficient to form the LTO/spinel crystalline, which is lower compared to by using TiO 2 anatase prepared from the solid state method as reported by Shin, et.al (J.W. Shin et al. 2012). The result from FTIR testing of both xerogel TiO 2 and the sintered product of mixture LiOH and anatase TiO 2 gives such indication of the formation of LTO and will be explained as follows. On the basis of analysis at Figure 4, the spectrum of broad transmittant band centered around 3300 cm 1 is attributed to stretching vibrations of hydroxyl group originating from Ti-OH (Vasconcelos et al. 2011). The other sources of hydroxyl group in the present work are from the remaining water and alcohol in the xerogel. Additional peaks are then observed at 2930 cm 1 (C-H 2 asymmetric stretching mode) and at 2867 cm 1 (C-H 2 symmetric stretching mode) of the alcoxide. The band around cm 1 is the characteristic of the stretch vibration of the C=O bonds. The small peak at 1343 cm -1 is contributed by the C-H bending from CH 3 group (Sui et al. 2006). There are also several small peaks at 1132, 1117, and 1022 cm -1 corresponding to Ti-O-C and the ending and bridging butoxyl groups of the alcoxide, respectively. The oxo bonds can be observed by the presence of wide bands below 800 cm -1 (Sui et al. 2006). Such oxo bonds come from Ti-O-Ti and TiO 6 octahedron stretching. As depicted in Figure 5, the result of FTIR of the sintered product shown that the spectrum around 2300 cm -1 prevails, which becomes stronger in line with increasing of the sintering temperature. The process during sintering that Li + source reacts with TiO 2 could be described as the migration of lithium ion into the nano-structure of TiO 2 to form LTO (Li 4 Ti 5 O 12 ) and such process is enhanced with increasing of the sintering temperatur at range of 550 to 750ºC. The results above agree well with the XRD analysis. As reported by Yan, H et al. (Yan et al. 2012), the Li 4 Ti 5 O 12 compound has two absorption peaks at cm -1 and cm -1, which are assigned to the stretching vibration of Ti-O bond and MO 6 (TiO 6 ) octahedron. The FTIR result of sintered product indicates that all spectra belongs to hydroxyl group from Ti-OH and water and alcohol, C-H 2 asymmetric stretching mode and C-H 2 symmetric stretching mode of the alcoxide are getting lower and almost disappear in the sintering temperature 750 ºC. The other spectra, such as the band around cm 1 of the C=O bonds, the peak at 1343 cm -1 of the C-H bending from CH 3 group and several peaks at 1132, 1117, and 1022 cm -1 from Ti-O-C are also decreasing at the same manner by increasing of the sintering temperature. This decreasing of the peaks is caused by decomposition of the related groups in the solid due to the temperature raised and the least residual organic compounds were found at the highest sintering temperature. In Figure 6 above, it is shown the FESEM image (10.000X) of the solid at the sintering temperature of 550, 650 and 750ºC, it can be seen that particles with various shapes and sizes sticking together formed irregular flaky agglomerate. This is because the grain bonding and particle growth during hydrothermal process and sintering and formed a dense particulate. All this mechanism leads to a low surface area as shown in the Table 2 above. The solid agglomerates of all products are with the size bigger than LTO KIST as shown in Figure 6(D). The tremendous image of Figure 5(D) shows the tiny particles with high level homogeneity in size and shape of LTO KIST which compose the porous sphere agglomerate. This structure leads to surface area much higher compared to LTO from the

8 present work and suggested that the surface templating method could be interesting to improve the product. Further, the dense of agglomerates and low surface area of the solid also suggest that mixing and hydrothermal processes of TiO 2 anatase with Li + ion source through aqueous solution LiOH impregnation method is not optimal, although the desired LTO is already obtained. 5. CONCLUSION The lithium titanate (Li 4 Ti 5 O 12 )/LTO has been successfully synthesized using hydrothermal method of TiO 2 xerogel prepared by sol-gel method and lithium hydroxide with calcination temperature 300ºC to form TiO 2 anatase and sintering temperature 750ºC to obtain the Li 4 Ti 5 O 12 )/spinel structure. The XRD-patterns shows all structure are readily crystallized. The lost of Li + source during processing is not superfluous, indicating that this TiO 2 xerogel and LiOH mixture by hydrothermal could initiate the reaction to form Li 4 Ti 5 O 12 and preventing the significant lost of Li + source. The effect of sintering temperature at the range of 550 ºC to 750 ºC shown that highest intensity of XRD peaks and FTIR spectra attributed to the LTO were found at the highest sintering temperature. Finally, the obtained high content of LTO (Li 4 Ti 5 O 12 ) with spinel crystalline structure at the sintering temperature 750 ºC could be used as active material in making the anode for Li-ion battery in further research and it is expected that such material will have a necessary performance compared to graphite. 6. ACKNOWLEDGEMENT The authors would like to thank the financial support from Hibah Pasca DRPM-UI (Direktorat Riset dan Pengabdian Masyarakat Universitas Indonesia) REFERENCES Augugliaro, V. et al., Comparison of Different Photocatalytic Systems for Acetonitrile Degradation in Gas solid Regime. Topics in Catalysis, 35(3-4), pp Bilecka, I. & Niederberger, M., New developments in the nonaqueous and/or nonhydrolytic sol gel synthesis of inorganic nanoparticles. Electrochimica Acta, 55(26), pp Byrappa, K. & Yoshimura, M., HANDBOOK OF HYDROTHERMAL TECHNOLOGY, A Technology for Crystal Growth and Materials Processing, Hintz, W. & Thomas, J., Mechanical Process Engineering - Particle Technology Agglomeration - Book Section. In Universitat Magdeburg. Hong, C.-H. et al., Effects of the starting materials and mechanochemical activation on the properties of solid-state reacted Li4Ti5O12 for lithium ion batteries. Ceramics International, 38(1), pp

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